1. Field of the Invention
The present invention relates generally to perovskite-type catalysts which are useful in carbon monoxide oxidation, hydrocarbon oxidation, nitrogen oxide reduction and oxidation of trapped soot particles. In addition, the present invention relates to perovskite-type materials displaying so-called giant magnetoresistance (GMR). Furthermore, the present invention relates to methods of making and using perovskite-type catalysts and materials.
2. Description of Related Art
Perovskite compositions are nominally designated as ABO3, in which A represents a rare earth metal, such as lanthanum, neodymium, cerium or the like, and B represents a transition metal such as cobalt, iron, nickel or the like. It is known in the art that perovskite-type materials are useful for the catalytic oxidation and reduction reactions associated with the control of automotive exhaust emissions. It is also known that perovskite materials (powders, single crystals and thin films) containing Mn on the B-site show giant magnetoresistance effect (GMR), such that on application of a magnetic field, the electrical resistivity of the material drops drastically due to a field-induced switching of the crystal structure. For this reason, GMR has attracted considerable attention for device applications, such as magnetic recording heads.
Several techniques have been used to produce perovskite-type catalyst materials for the treatment of exhaust gases from internal combustion engines. The ability of such materials to effectively treat internal combustion exhaust gases depends on the three-way activity of the material, i.e., the capability for nitrogen oxide reduction, carbon monoxide oxidation and unsaturated and saturated hydrocarbon oxidation. The following patents describe such materials and techniques in the three-way catalytic application: U.S. Pat. Nos. 3,865,752; 3,865,923; 3,884,837; 3,897,367; 3,929,670; 4,001,371; 4,049,583, 4,107,163; 4,126,580; 5,318,937. In particular, Remeika in U.S. Pat. No. 3,865,752 describes the use of perovskite phases incorporating Cr or Mn on the B-site of the structure showing high catalytic activity. Lauder teaches in U.S. Pat. No. 4,049,583 (and U.S. Pat. No. 3,897,367) the formation of single-phase perovskite materials showing good activity for CO oxidation and NO reduction. Tabata in U.S. Pat. No. 4,748,143 teaches the production of single-phase perovskite oxidation catalysts where the surface atomic ratio of the mixed rare earth elements and the transition metal is in the range of 1.0:1.0 to 1.1:1.0. The rare-earth component can be introduced using a mixed rare-earth source called xe2x80x9cLex 70xe2x80x9d which has a very low Ce content. Tabata further teaches in U.S. Pat. No. 5,185,311 the support of Pd/Fe by perovskites, together with bulk ceria and alumina, as an oxidation catalyst. The perovskite is comprised of rare earths on the A-site and transition metals on the B-site in the ratio 1:1.
In addition to these patents there are numerous studies reported in the scientific literature relating to the fabrication and application of perovskite-type oxide materials in the treatment of internal combustion exhaust emissions. These references include Marcilly et al., J. Am. Ceram. Soc., 53 (1970) 56; Tseung et al., J. Mater. Sci., 5 (1970) 604; Libby, Science, 171 (1971) 449; Voorhoeve et al., Science, 177 (1972) 353; Voorhoeve et al., Science, 180 (1973); Johnson et al., Thermochimica Acta, 7 (1973) 303; Voorhoeve et al., Mat. Res. Bull., 9 (1974) 655; Johnson et al., Ceramic Bulletin, 55 (1976) 520; Voorhoeve et al., Science, 195 (1977) 827; Baythoun et al., J. Mat. Sci., 17 (1982) 2757; Chakraborty et al., J. Mat. Res., 9 (1994) 986. Much of this literature and the patent literature frequently mention that the A-site of the perovskite compound can be occupied by any one of a number of lanthanide elements (e.g., Sakaguchi et al., Electrochimica Acta, 35 (1990) 65). In all these cases, the preparation of the final compound utilizes a single lanthanide, e.g., La2O3. Meadowcroft, in Nature, 226 (1970) 847, refers to the possibility of using a mixed lanthanide source for the preparation of a low-cost perovskite material for use in an oxygen evolution/reduction electrode. U.S. Pat. No. 4,748,143 refers to the use of an ore containing a plurality of rare-earth elements in the form of oxides for making oxidation catalysts.
In addition to the above-mentioned techniques, other techniques have been developed for the production of perovskite materials containing Mn on the B-site which show giant magnetoresistance effect (GMR). Such materials are generally made in the forms of powders, single crystals and thin films. A common technique is the growth of single-crystal from a phase-pure perovskite source (see, for example, Asamitsu in Nature, 373 (1995) 407). All such techniques use a phase-pure perovskite compound with a single lanthanide on the A-site, in addition to an alkaline earth dopant. An example of such phase-pure perovskite compounds is La1xe2x88x92xSrxMnO3.
It is also known in the art that it is difficult and expensive to prepare individual rare-earth compounds such as individual lanthanides. The cost is high for making perovskite-type materials with a single lanthanide on the A-site. Therefore, a need exists for using low-cost starting materials to manufacture inexpensive catalyst materials, simultaneously having high temperature stability and high three-way activity for use in conversion of CO, hydrocarbons and oxides of nitrogen in modem gasoline-powered automobiles. A need also exists for the manufacture of bulk materials, thin films and single-crystals of materials showing GMR, using inexpensive starting materials.
It is an object of the present invention to provide improved catalyst materials with three-way activity, manufactured from inexpensive starting components. It is also an object of the present invention to provide a perovskite-type metal oxide compound having giant magnetoresistance effect. It is further an object of the present invention to provide a method of making and using improved catalyst materials and the perovskite-type metal oxide compounds of the present invention.
Accordingly, one object of the present invention is to provide a perovskite-type catalyst consisting essentially of a metal oxide composition. The metal oxide composition is represented by the general formula Aaxe2x88x92xBxMOb, in which A is a mixture of elements originally in the form of single phase mixed lanthanides collected from bastnasite; B is a divalent or monovalent cation; M is at least one element selected from the group consisting of elements of an atomic number of from 22 to 30, 40 to 51, and 73 to 80; a is 1 or 2; b is 3 when a is 1 or b is 4 when a is 2; and x is a number defined by 0xe2x89xa6x less than 0.7.
In a preferred embodiment, the single phase perovskite-type materials of the present invention have a formula A1xe2x88x92xBxMO3, and preferably x is about 0 to 0.5.
In another preferred embodiment, the single phase materials of the present invention are perovskite-type materials having a formula A2xe2x88x92xBxMO4.
Another object of the present invention is to provide a perovskite-type metal oxide compound represented by the general formula Aaxe2x88x92xBxMOb, in which A is a mixture of elements originally in the form of single phase mixed lanthanides collected from bastnasite; B is a divalent or monovalent cation; M is at least one element selected from the group consisting of elements of an atomic number of from 22 to 30, 40 to 51, and 73 to 80; a is 1 or 2; b is 3 when a is 1 or b is 4 when a is 2; and x is a number defined by 0xe2x89xa6x less than 0.7. The perovskite-type metal oxide compound of the present invention contains Mn on its B-site.
The present invention also provides a catalytic converter. The catalytic converter comprises:
(a) a perovskite-type catalyst comprising a metal oxide composition represented by the general formula:
A1xe2x88x92xBxMO3
xe2x80x83wherein
A is a mixture of elements originally in the form of a single phase mixed lanthanide collected from bastnasite;
B is a divalent or monovalent cation;
M is at least one element selected from the group consisting of elements of an atomic number of from 22 to 30, 40 to 51, and 73 to 80;
x is a number defined by 0xe2x89xa6x less than 0.5; and
(b) a solid structure for supporting the catalyst.
In a preferred embodiment, the catalytic converter also comprises a carrier powder. The peroviskite-type catalyst may be in a bulk form or in the form of a dispersion.
Another object of the present invention is to provide a method of preparing a perovskite-type catalyst consisting essentially of a metal oxide composition having component elements represented by the general formula Aaxe2x88x92xBxMOb, in which A is a mixture of elements originally in the form of single phase mixed lanthanides collected from bastnasite; B is a divalent or monovalent cation; M is at least one element selected from the group consisting of elements of an atomic number of from 22 to 30, 40 to 51, and 73 to 80; a is 1 or 2; b is 3 when a is 1 or b is 4 when a is 2; and x is a number defined by 0xe2x89xa6x less than 0.7.
The method comprises forming a homogeneous mixture of a single phase mixed lanthanide salt collected from bastnasite and respective salts or oxides, of elements B and M and forming a perovskite-type metal oxide composition from said homogeneous mixture.
A further aspect of the present invention provides a method of making a catalytic converter of the present invention. The method comprises:
(a) providing a perovskite-type catalyst comprising a metal oxide composition represented by the general formula:
A1xe2x88x92xBxMO3
xe2x80x83wherein
A is a mixture of elements originally in the form of a single phase mixed lanthanide collected from bastnasite;
B is a divalent or monovalent cation;
M is at least one element selected from the group consisting of elements of an atomic number of from 22 to 30, 40 to 51, and 73 to 80;
x is a number defined by 0xe2x89xa6x less than 0.5;
(b) providing a support structure having a surface;
(c) forming a stable slurry suspension of the perovskite-type catalyst; and
(d) depositing the suspension of the perovskite-type catalyst on the surface of the support.
In one embodiment of the present invention, the perovskite-type catalyst may be in a bulk form or in the form of a dispersion. A carrier powder may be mixed with the bulk perovskite-type catalyst to form the slurry suspension and then be deposited on the surface of the support. Alternatively, a carrier powder may be used to form a perovskite-type catalyst in the form of a dispersion.
The present invention is further defined in the appended claims and in the following description of preferred embodiments.